Liquid crystal displays (LCDs (Liquid Crystal Displays)), which are one of the typical, conventionally-used devices for thin panels, have widely been used for a monitor and others of a personal computer and a handheld information terminal device, because of their advantages such as low electric power consumption, small size, and light weight. In recent years, the LCDs have also widely been used for television-specific applications, so that conventional cathode ray tube-based televisions are being replaced by liquid crystal display televisions.
Recently, electroluminescence EL (Electroluminescence) displays, which have overcome the problems of the LCDs, such as a limitation of a field of view and a contrast, a low response speed in displaying moving images, have also been used as next-generation devices for thin panels. The electroluminescence EL displays use a light emitter such as an EL element in a pixel display unit, and thus have characteristics such as self light-emitting properties, wide field-of-view performance, high contrast performance, and fast responsiveness, which have not yet been achieved by the LCDs.
For a TFT (Thin Film Transistor) used for the displays as described above, a MOS (Metal Oxide Semiconductor) structure that uses a semiconductor thin film is often used. Examples of the structures of the TFT include an inverted staggered type and a top gate type. Examples of the semiconductor thin films used for the TFT include an amorphous type and a polycrystalline type. The structures of the TFTs and the types of semiconductor thin films are selected as appropriate depending on intended purpose and performance of a display. For a small-size display, a polycrystalline-type semiconductor thin film is often used because size reduction in TFT can increase an aperture ratio of a display region.
As one of the methods of manufacturing a polycrystalline semiconductor thin film, there has been known a method of forming a polycrystalline semiconductor thin film by irradiating an amorphous semiconductor thin film formed on a substrate with a laser beam. It is known that a polycrystalline semiconductor thin film made of crystals having a size of approximately 0.2-1.0 μm can be formed with this method.
For example, according to Japanese Patent Laying-Open No. 2003-017505, a polycrystalline silicon layer (polycrystalline semiconductor thin film) is formed by irradiating an amorphous silicon layer (amorphous thin film) formed on a substrate with a laser beam.
Further, there has also been proposed a method of irradiating an amorphous semiconductor thin film with a laser beam to form a polycrystalline semiconductor thin film having a desired crystal size.
For example, according to Japanese Patent Laying-Open No. 2005-244230, a light reflective layer is formed at a portion of an amorphous silicon layer (amorphous thin film), and then a laser annealing process with use of an excimer laser having a wavelength of 308 nm or a green laser having a wavelength of 532 nm is conducted. With this process, a polysilicon layer (polycrystalline semiconductor thin film) having a relatively large polysilicon grain size (a diameter of approximately 0.3-0.4 μm) is formed in a portion that is not covered with the light reflective layer, and a polysilicon layer having a relatively small polysilicon grain size (a size smaller by approximately 0.1 μm in diameter) is formed in a portion of the amorphous silicon layer that is covered with the light reflective layer.
There has also been known a method of manufacturing a TFT that uses a polycrystalline semiconductor thin film formed as described above. Specifically, a gate insulating film made of silicon oxide or the like is initially formed on the polycrystalline semiconductor thin film. On the gate insulating film, a gate electrode is formed. The gate insulating film is used as a mask to introduce impurities such as phosphorus or boron into the polycrystalline semiconductor thin film, to thereby form source/drain regions. Subsequently, an interlayer insulating film is formed to coat the gate electrode and the gate insulating film. Subsequently, a contact hole reaching each of the source/drain regions is made in the interlayer insulating film and the gate insulating film. Through the contact hole, a metal film is formed to connect to each of the source/drain regions. The metal film is patterned to form each of source/drain electrodes. Consequently, the TFT is manufactured.
By forming a pixel electrode or a self light-emitting element to be connected to the drain electrode of the TFT described above, a display that uses the TFT is manufactured.
Furthermore, studies have been made on the correspondence between irradiation with a laser beam and the state of how crystal grains subjected to this irradiation are arranged. For example, according to Non-Patent Document 1, a silicon substrate is irradiated with 6000 pulses of a laser beam having linearly-polarized light and having a wavelength of 532 nm. After the silicon substrate is rotated by 90°, the laser beam is applied thereto once again. By doing so, it is said that a two-dimensional surface shape having a period of approximately 550 nm can be obtained on the silicon substrate.
Furthermore, Japanese Patent Laying-Open No. 2005-072165 shows that, when an amorphous silicon film on a glass substrate is irradiated with a laser beam, it is possible to obtain a polycrystalline silicon film that has concavities and convexities having a period of λ/n (1+sin θ) on its surface, where the laser beam has a wavelength of λ, an atmosphere has a refractive index of n, and an angle formed by the laser beam and the normal to a plane of the amorphous silicon film is θ.    Patent Document 1: Japanese Patent Laying-Open No. 2003-017505    Patent Document 2: Japanese Patent Laying-Open No. 2005-244230    Patent Document 3: Japanese Patent Laying-Open No. 2005-072165    Non-Patent Document 1: Y. Nakata, A. Shimoyama, and S. Horita, “Crystallization of an a-Si film by a Nd: YAG pulse laser beam with linear polarization”, Digest of Technical Papers 2000 International Workshop on Active-Matrix Liquid Crystal Displays (AM-LCD2000), pp. 265-268